Many missiles employ solid fuel rocket motors for propulsion. In most instances, the rocket motor is also the main airframe of the flight vehicle. Therefore the rocket motor often has several attached parts, e.g., fins, explosives, guidance electronics. In some cases it is necessary to attach parts to the pressure vessel. The current method of attaching parts to rocket motors is through use of a series of clips welded to the rocket motor chamber. Welding is typically accomplished by conventional electric conduction welding or electron beam welding. However, each welding technique is deficient when it comes to attaching parts to rocket motors.
In particular, electron beam welding is sensitive to stray electromagnetic fields. Accordingly, if the rocket motor being welded is not maintained in a vacuum, the electron beam alignment is likely to be affected by these stray magnetic fields. Unfortunately, the size requirements of such a vacuum chamber are not practical in the case of rocket motors due to costs and the time required to evacuate such a chamber.
Furthermore, welding of the conventional clips can only be accomplished in the circumferential direction with respect to the chamber because axial welding weakens the pressure vessel. FIG. 1(a) is a perspective view of a solid fuel rocket motor chamber 10 with a series of conventional clips 11 individually welded along the circumferential interface 13 of each clip 11. FIG. 1(b) depicts a fin 15 attached to the welded clips 11. The welding of clips 11 in the circumferential direction requires extended surfaces at the base of the clip 11 (thereby forming circumferential interface 13) to provide a sufficient weld area for strength. As a result, the weight of the clip is increased accordingly. Additional strength for attachments can be accomplished only through the use of larger clips or closer spacing of smaller clips. However, closer spacing is limited by the necessity to reach the weld area with the welding rod.
The welding of clips on rocket motor chambers using electric conduction welding creates a heat affected zone which necessitates further heat treatment of the assembled unit. The conventional weld, such as a gas tungsten arc weld, penetrates approximately 25-50 percent of the rocket motor chamber's case. This amount of weld penetration causes heat affected zones completely throughout the rocket motor case. The resulting thermal expansion and subsequent contraction can cause permanent distortion of the assembled unit In addition, these welding methods cannot be applied to rocket motor chambers which are fully loaded with live propellant. The heat generated by the welds could ignite the propellant. For the same reason, repair of damaged parts or clips cannot be effected on fully loaded rocket motors.
Because the clip can only be welded in the circumferential direction, a void is formed along the longitudinal interface 17 of each clip 11. In practice it has not been possible to completely protect this void from corrosion with special coatings, e.g. phosphate, epoxy. Furthermore, when the rocket is in flight, air flow over the rocket motor chamber 10, indicated by the arrows in FIG. 1(b), is interrupted causing a variety of problems. First, air flow interruption occurs along the longitudinal voids formed along each longitudinal interface 17. Secondly, air flow is interrupted by the spaces between adjacent clips 11. Finally, since the air flow will be aerodynamically heated, the interior of fin 15 may need to be thermally protected.